Methods for increasing the sensitivity of silver halide crystals to light are well known and discussed in detail in "The Theory of the Photographic Process," 4th edition, edited by T. H. James, pp. 149-169. The most common methods of chemical sensitization are classified as sulfur sensitization, gold sensitization and reduction sensitization. The first two classes are commonly combined to get the highest increase in sensitivity. Less commonly, reduction sensitization is combined with one of the other methods. James points out that it is difficult to successfully combine gold and reduction sensitization. There are many published claims for other methods of sensitization, especially using various noble metals such as Platinum, Palladium or Iridium. However, in most emulsions based on gelatin as the colloid, the simple salts or complexes of these metals do not increase the sensitivity because the gelatin forms strong complexes with the metal which prevents deposition of the metal onto the crystal surface. If the colloid has no reactive sites for the noble metal to form complexes, such as polyvinyl alcohol, then the metals do act as sensitizers.
It is also known that chemical sensitization can influence the reciprocity failure characteristics of the emulsion. Sulfur sensitization increases high intensity failure, while sulfur plus gold reduces this failure. Reduction sensitization or a reducing treatment before exposure tend to eliminate low intensity failure.
In the formulation of photographic products, it is often necessary to control the reciprocity failure while also increasing the overall sensitivity by using combinations of sensitizers and novel sensitizers.